The human immunodeficiency virus (HIV) employs an array of properties that enable it to evade antibody neutralization. Rational vaccine design has been limited due to the insufficient structural understanding of the HIV envelope glycoprotein (Env), the primary target for neutralizing antibodies. Advances in structural determination have been hindered due to the presence of flexible hypervariable loops and the high carbohydrate content of this protein and the arrangement of the full Env complex remains controversial. The nature of the glycan shield, thought to mask conserved sites, is also of key interest as it mediates immune escape. Furthermore the binding of host cell CD4 receptor is known to induce massive conformational changes in Env, but a detailed examination of these changes has yet to be addressed. This proposal aims to combine two complementary approaches, small-angle X-ray scattering (SAXS) with ab initio shape reconstruction and hydrogen-deuterium exchange (HXMS) with mass spectrometry analysis to address several questions regarding the architecture of trimeric Env. The extensive catalog of constructs available from the Hu lab enables comparative studies of Env ranging from monomeric subunits to trimeric complexes, in the context of the same isolate. Identification of the trimeric interface, conformational changes induced upon receptor binding, and epitope occlusion by variable loops and glycans can be addressed with these techniques. The interaction of several well-characterized monoclonal antibodies directed against the CD4 binding site will also be compared to address the question of how conformational changes induced by antibody binding differ from those resulting from CD4 binding, and whether there are potentially correlates with neutralizing potency. Understanding these properties of Env will assist current efforts to identify targets for development of broadly neutralizing antibodies, necessary for an effective vaccine.